Process for producing standard gas mixture and apparatus for producing the same

ABSTRACT

The present invention provides a process for producing standard gas mixtures which entails purifying a raw gas using a gas purifier (3) such as a known two stage gas purifier system, thereby generating a high-purity diluent gas; 
     mixing the high-purity diluent gas and at least one high-concentration standard gas, by generating a medium-concentration gas mixture; 
     dividing the medium-concentration gas mixture into a first flow and a second flow; 
     selectively mixing the first flow with a sample gas or a high-purity diluent gas or a mixture thereof, thereby obtaining a standard low-concentration gas mixture; and 
     controlling the pressure of the sample gas, the second flow of the medium-concentration gas mixture and the standard low concentration gas mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to manufacture of standard gas mixturesand, more particularly, to a process and an apparatus for producingstandard gas mixtures for testing, calibrating, . . . highly sensitiveanalytical instruments such as an atmospheric-pressure ionization massspectrometer (APIMS).

2. Description of the Background

A variety of gases are used in manufacturing semi-conductor devices suchas LSIs. These gases contain impurities. The impurities have an adverseinfluence on the characteristics of the LSIs. Hence, it is demanded thatthe gases be as pure as possible. This demand grows stronger, along withthe increase in the integration density of LSIs. To meet this demand, ahigh-accuracy and reliable analysis of gases is required.

The techniques commonly used for analyzing such gases for determiningthe impurity contents thereof are: gas chromatography, gaschromatography-mass spectroscopy, and Fourier transformed infraredspectroscopy. The detection limit of these techniques are, however, 1 to10 ppb at best. In view of this, these analytical techniques cannot besaid to determine the impurity content of the gases as sensitively as isrequired for example in the manufacture of LSIs.

Known as apparatus for producing standard gas mixtures for use in thevarious analytical instruments is a standard gas generator which dilutesa high-concentration gas mixture containing the impurity to be analyzedwith a so-called zero gas in one or more stages, thereby generating astandard gas mixture under a predetermined pressure at a predeterminedflow rate. This apparatus is very useful in producing standard gasmixture to be used to obtain calibration curves. However, it cannot beemployed in the internal standard method or the standard additionmethod, wherein a known amount of a high-concentration standard gas isadded directly to a sample gas. Nor can the apparatus be used to producea standard gas mixture which contains multiple component gases.

Therefore, it has been demanded that a process and an apparatus bedeveloped which can produce these various gas mixtures.

More generally, most of the following functions are required in thedaily practice of gas analysis:

Introduce the gas to be analyzed in a controlled way into the analyzingdevice;

Add to the gas to be analyzed, calibrated amounts of gaseous species inorder to apply the standard addition and internal standard generationmethods;

Introduce a blanc or zero gas into the analyzing device;

Add to the zero gas calibrated amounts of gaseous species in order togenerate controlled mixtures facilititating the calibration of theanalyzing device;

Dilute in a controlled way the gas to be analyzed by the zero gas beforeintroduction of the mixture into the analyzing device. This last pointis particularly relevant for very polluted, or toxic and/or corrosivegases.

SUMMARY OF THE INVENTION

The process and apparatus according to the present invention makes itpossible to complete the above functions in a simple and efficient waywhich is specially important when doing ultratrace level analysis.

The object of the present invention is to provide a process and anapparatus for producing standard gas mixtures, each of which containsdifferent gases and can be used in either the internal standard methodor the standard addition method.

According to the present invention, there is provided a process forproducing standard gas mixtures, comprising the steps

A process for producing standard gas mixtures, comprising the steps of:

Controlling the pressure of a raw gas;

Purifying said raw gas, thereby generating a high-purity diluent gas;

Controlling the flow rate of at least one high concentration standardgas;

Mixing the high-purity diluent gas and said at least onehigh-concentration standard gas, thereby generating amedium-concentration gas mixture;

Dividing the medium-concentration gas mixture into a first flow and asecond flow;

Mixing the first flow with a sample gas or the high purity diluent gas,or a mixture thereof, thereby obtaining a standard low-concentration gasmixture;

Controlling the pressure of the sample gas;

Controlling the pressure of said second flow of said mediumconcentration gas mixture;

Controlling the pressure of said standard low concentration gas mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing a process of producinglow-concentration gas mixtures, in which a high-concentration standardgas is diluted in two stages.

FIG. 2 is a flow diagram showing a process of producing alow-concentration gas mixture, in which a high-concentration standardgas is diluted in n stages.

FIG. 3 illustrates another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Other embodiments of the process according to the invention are carriedout according to, among others, any one of claims 2 to 13.

Further, according to this invention, there is provided an apparatus forproducing standard gas mixtures comprising:

a plurality of gas sources including a source of raw gas, a source ofsample gas and sources of different high-concentration standard gases;

A manifold for gathering the high concentration standard gases from thesources of high concentration standard gases and delivering a highconcentration standard gas mixture;

purifying means for purifying the raw gas supplied from said source ofraw gas, thereby generating a high purity diluent gas,

dividing means for dividing said high purity diluent gas in a first andsecond portion;

mixing means for mixing said first portion of said high purity diluentgas with said high concentration standard gas mixture,

dividing means for dividing the medium concentration gas mixture into afirst flow and second flow;

means for mixing the second portion of the high purity diluent gas andthe sample gas and generating a diluent gas which is either the highpurity diluent gas or the sample gas or a mixture thereof;

means for mixing said first flow of said medium concentration gasmixture and said diluent gas generating a standard gas mixture.

The apparatus according to the present invention may further comprisemeans according to any one of claims 16 to 25.

According to a further embodiment of the invention there is provided anapparatus for producing standard gas mixtures, comprising:

a plurality of gas sources including a source of raw gas and sources ofdifferent high-concentration standard gases;

a manifold for gathering the high-concentration standard gases from thesources of high-concentration standard gases and delivering ahigh-concentration standard gas mixture;

a source of sample gas;

a sample gas passage connected to said source of the sample gas;

means for controlling the pressure of the raw gas;

means for controlling the pressure of the sample gas;

purifying means for purifying the raw gas supplied from said source ofraw gas, thereby generating a high purity diluent gas;

means for dividing said high purity diluent gas in a first and secondportions, said second portion flowing through first restriction means;

mixing means for mixing the first portion of the high-purity diluent gaswith the high-concentration standard gas mixture supplied from saidmanifold, thereby generating an about homogeneous medium-concentrationgas mixture;

means for dividing the medium-concentration gas mixture into a firstflow and second flow, said first flow being supplied through secondrestricting means;

means for controlling the flow rate of the second portion of the highpurity diluent gas, said means having a closed position wherein no gasflows, and an open position wherein the flow rate of said diluent gascan be controlled;

means for controlling the flow rate of the sample gas, said means havinga closed position wherein no sample gas flows, and an open positionwherein the flow rate of the sample gas can be controlled. Thirdrestriction means receiving the second portion of the high puritydiluent gas and/or the sample gas and generating a diluent gas;

means for mixing said first flow of said medium-concentration gasmixture and said diluent gas for generating a standard low-concentrationgas mixture;

means for controlling the pressure of the second flow of saidmedium-concentration gas mixture;

means for controlling the pressure of said standard low-concentrationgas mixture.

In the present invention, the ratio of one flow of the low-concentrationgas mixture to another flow of diluent gas and/or of sample gas, can beset at any desired value.

In the process and the apparatus according to the present invention, asingle unit may comprise multiple gas sources including a source of rawgas, a source of a sample gas, and sources of differenthigh-concentration gases. The high-concentration standard gas suppliedfrom any high-concentration standard gas source can be diluted twice,first with a diluent gas obtained by purifying the raw gas supplied fromthe raw gas source, and then with the diluent gas and/or the sample gassupplied from the sample gas source. Therefore, when the valves coupledto the lines connecting these gas sources are operated, various standardgas mixtures can be produced.

When the line for supplying the sample gas, for example, is closed, thehigh-concentration standard gas mixture is diluted with the zero gasonly, thereby preparing a standard gas mixture. When the branched lineof the zero gas for the second dilution is closed, thehigh-concentration standard gas mixture is diluted, first with the zerogas and then with the sample gas, thereby producing a standard gasmixture. Hence, the process and the apparatus according to the presentinvention can produce standard gas mixtures which can be utilized in theinternal standard method and the standard addition method, thus makingcalibration curves which can be used in analyzing the sample gas.

Moreover, two or more sources of high-concentration gas mixtures, whichare used in the present invention, can be selected by operating thevalves coupled to these sources. When various kinds ofhigh-concentration standard gas mixtures are supplied from thesesources, the concentrations of the various gases forming the standardgas mixture, i.e., the final product, can be changed independently ofone another. In order words, the process and the method according to thepresent invention can produce a variety of standard gas mixtures whichare useful particularly in determing the influence imposed on theanalytical results of one gas component from other components.

The basic concept of the present invention is to provide a process andrelated apparatus wherein, as soon as the high purity diluent gas isgenerated, no further contaminants such as particles and/or gaseousimpurities, . . . are added during the further mixing and/or dilutingsteps of the process. That means that all the devices used for carryingout those steps are able to generate no additional contaminants. Thosedevices are generally selected among pipes, such as electropolishedpipes, and restriction means, such as needle valves (controllableflowrate), calibrated orifices, small diameter pipes, with appropriatediameters as well as appropriate ratios of diameters when differentflow-rates and pressures have to be handled in different lines, theselection of the appropriate ratio of diameters being well-known by theman skilled in the art.

No devices which might potentially be contaminant sources are used inthe zero gas lines or the mixing lines in the process or apparatusaccording to the present invention. Orifices or needle valves, which arenot contaminant sources, are generally used in the zero gas lines ormixing lines. In the conventional prior art process and apparatus, massflow controllers and pressure regulators are placed in the zero gaslines and the mixing lines in order to control the flow rates andpressures of the low-concentration gas mixture. According to the presentinvention, mass flow controllers and pressure regulators are placedupstream of the gas-purifying means, in the high-concentration standardgas lines, the contamination of which is not so problematical, and inthe gas-venting lines which are downstream the gas lines of the systemaccording to the invention which are thus prevent from contamination.This specific arrangement of the mass flow controllers and the pressureregulators is based on the inventors' finding that the controllers andregulators can control correctly the flow rate and pressure of thelow-concentration gas mixture.

In the body of the present specification, various steps are definedwhich may have the following meanings according to the invention:

Controlling the pressure of the raw gas is preferably carried out eitherbefore the purification step, by means of a pressure regulator or afterthe purification step, by means of a back pressure regulator.

Controlling the flow rate of a high concentration standard gas may bepreferably accurately carried out either by means of a mass flowcontroller or a neddle valve associated to a pressure regulator.

Controlling the pressure of the low concentration gas mixture, i.e. themixture adapted to flow in the analyzer, such as APIMS, is carried outpreferably:

with a back pressure regulator connected at the end of the pipedelivering the low concentration gas mixture, thus avoiding about anycontamination of the pipe.

with the analysis apparatus itself which is sometimes able to controlitself said pressure (e.g. when the analysis is carried out atatmospheric pressure).

with a pressure regulator connected at the output of the analysisapparatus, thus providing no contamination of the low concentration gasmixture.

In the present specification the terms high purity diluent gas or zerogas are equivalent. Such high purity gas is obtained by well-knowntechniques such as catylisis, chemical conversion, gettering, ambienttemperature physical absorption, cryogenic absorption, filtering withmolecular sieves, . . . or a combination of those methods, in order toremove about any impurities such as particles and gaseous impurities, .. .

Various appropriate methods are disclosed for example in the article ofF. W. Giaccobbe and G. S. Khan, both of American Air Liquide Inc.,entilted "Production of ultra high purity nitrogen"--Solid stateTechnology--July 1987--.

FIG. 1 is a flow diagram illustrating how an apparatus according to theinvention operates to produce standard gas mixtures. As is shown on thisFIG. 1, raw gas source 1 is connected to pressure regulator 2, which inturn is coupled to purifier 3. The outlet port of purifier 3 is coupledto branching tube 4 which has two outlets ports. The first outlet portof tube 4 is connected to a needle valve 5. The needle valve 5 isconnected to a mixing chamber 6. The second outlet port of tube 4 iscoupled to a stop valve 9b, which in turn is connected to one of the twoinlet ports of branching tube 10. Sample gas source 7 is connected to apressure regulator 8. The pressure regulator 8 is coupled to a stopvalve 9a. The stop valve 9a is connected to the other inlet port of thebranching tube 10. The outlet port of this tube 10 is coupled to aneedle valve 11a. A plurality of high-concentration standard gas mixturesources 12 and 13 are connected respectively to a pressure regulator 14.These regulators 14 are coupled respectively to mass flow controllers15, and mass flow controllers 15 are connected to a manifold 16 whichcan be an integrated valve (a double 3 ways valve as detailed forexample in reference "Nikkei Microdevice, July 1987, challenge to ppt,T. Ohmi"). The outlet port of the manifold 16 is connected to the mixingchamber 6.

The outlet port of the mixing chamber 6 is coupled to the branching tube17 having two outlets ports. The first outlet port of tube 17 isconnected to back-pressure regulator 18, which in turn is coupled toflowmeter 19, which in turn is connected to a gas-discarding port. 20.The second outlet port of the branching tube 17 is coupled to a needlevalve 11b. The valve 11b is connected to a branching tube 21 whichconnects the outlet of the needle valve 11a to an analytical instrument22. The outlet port of the instrument 22 is coupled to a back-pressureregulator 23, which in turn is connected to a flowmeter 24, which inturn is connected to a gas-dicarding port 25.

When the valves coupled to the lines connecting the gas sources of theapparatus described above are operated selectively, various standard gasmixtures can be produced. It will be explained hereafter how thisapparatus can realize the various functions referred to hereabove.

The raw gas is supplied from material gas source 1 to the purifier 3through the pressure regulator 2. Purifier 3, (such as two stagespurifier system FP-S and FP-W, sold by Kinki Reinetsu K. K.) purifiesthe raw gas, thus forming a so called zero gas. Meanwhile, the samplegas is supplied from the sample gas source 7 to the pressure regulator8.

The various functions disclosed hereabove and which are required in thedaily practice of gas analysis will now be disclosed in more details:

Function 1 Introduce the gas to be analyzed in a controlled way into theanalyzing device

When the valve 9b and the restriction 11b are closed while the valve 9aand the restriction 11a are opened, the sample gas is directlyintroduced in the analyser, realizing thus mentioned function 1.

Function 2 Add to the gas to be analyzed, calibrated amounts of gaseousspecies in order to apply the standard addition and internal standardgeneration methods

When the valve 9b is closed, and the stop valve 9a and the needle valve11a are opened, the high-concentration standard gas mixture is dilutedfirst with the zero gas and then with the sample gas, generating astandard gas mixture which is suitable to draw calibration curves foruse in both the internal standard method and the standard additionmethod, thus accomplishing function 2.

Function 3 Introduce a blanc or zero gas into the analyzing device

When the valve 9a and the supply of high concentration gas mixture frommanifold 16 is cut off there are three possible ways of implementingfunction 3, i.e. the introduction of zero gas (or blanc) into theanalyzer:

Closing 9b and opening 5 and 11b,

Closing 5 and 11b while opening 9b and 11a,

Opening 5, 9b, 11a, 11b.

Function 4 Add to the zero gas calibrated amounts of gaseous species inorder to generate controlled mixtures facilititating the calibration ofthe analyzing device

This function can be carried out for example in one step or two steps:

a--one step

When the stop valves 9a and 9b and the needle valve 11a are closed,whereas needle valve 5 is opened, the zero gas is supplied from thepurifier 3 to the mixing chamber 6 through the needle valve 5. The zerogas is, therefore, mixed with the high-concentration standard gasmixtures supplied from the plurating of sources 12, . . . 13 through themixing chamber 6, thus generating a standard gas mixture. The standardgas mixture is divided into two streams by the branching tube 17. Thefirst stream, flows through the back-pressure regulator 18 and theflowmeter 19 and is finally discarded via the gas discarding port 20.The second stream of the standard gas mixture is introduced into theanalytical instrument 22 via the needle valve 11b and the branching tube21. This standard gas mixture has been prepared by diluting thehigh-concentration gas mixture only one time with the zero gas. It iscalled a one stage dilution gas mixture. This method is, on the otherhand, another way to carry out function 4, when the stop valve 9a isclosed,

b--two steps

The needle valve 5, the stop valve 9b, and the needle valves 11a and 11bare opened, and the stop valve 9a is still closed. The zero gas from thepurifier 3 is divided into two streams by the branching tube 4. As aresult, the high-concentration standard gas mixture from the manifold 16is diluted twice with the zero gas: a first time in the mixing chamber 6and a second time in the branching tube 21, thus producing a standardlow-concentration gas mixture. This constitutes the second way, in twosteps to carry out the function 4 of introduction of calibratedmixtures. The possibility to carry out function 4 by the one-stepdilution method as well as by the two-step dilution method permits todetermine the ratio of the flows involved in the second step of thetwo-step dilution. With an analyzer and by production of the sameamplitude of signal for a certain component added in the one-step or thetwo-steps methods, it is possible to determine the ratio of the flowsadded in the second dilution step. The ratio between the flows throughthe mass flow controller 15 for those two set-ups (one step and twosteps dilution) is the ratio of dilution in the second step.

Function 5

Dilute in a controlled way the gas to be analyzed by the zero gas beforeintroduction of the mixture into the analyzing device. This last pointis particullarily relevant for very polluted, or toxic and or corrosivegases.

Closing 9b and opening 5, 11b, 11a and 9a allows to dilute the samplegas with the zero gas, thus realizing function 5.

In the present embodiment, either the high-concentration standard gasmixture supplied from one source only or those from a plurality ofsources such as 12, . . . 13, can be used to produce multiple standardgas mixtures.

FIG. 2 is a second embodiment of the invention, the same device as thoseof FIG. 1 bearing the same references. Valves 9a and 9b are replaced byrestrictions 11a and 11c respectively, to control the pressure and theflowrate of the sample gas in combination with pressure regulators 8 and23 as explained before and of the second portion of the high high puritydiluent gas. Thus a ratio of flowrates of each of these gases is fixedand cannot be modified without changing the characteristics of therestrictions. A mixture of those two gases is then made at theT-jonction 110, said mixture being diluted (or mixed) with the firstflow of the medium concentration gas mixture coming from the needlevalve 116.

FIG. 3 is another embodiment of the invention where the second portionof the high purity diluent gas is added through needle valve 11c to themixture flowing from the outlet of the branching tube 21, but beforeintroduction of the same in the analyzer 22.

As described hereabove it is possible according to the present inventionto produce a variety of standard gas mixtures by operating the valvescoupled to the lines for supplying the zero gas, the sample gas, thehigh-concentration standard mixture gases, and their mixtures thereof.The standard gas mixtures, thus produced, are suitable to drawcalibration curves for use in the internal standard method and thestandard addition method.

The apparatus according to the invention has multiple sources ofhigh-concentration standard gas mixtures. Therefore, any possiblecombination of these gas mixtures can be used, merely by operatingvalves connected to the outlet ports of these gas mixture sources.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A process for producing standard gas mixturescomprising:a) controlling the pressure of a raw gas using pressurecontrolling means upstream of gas purifying means; b) purifying the rawgas using the gas purifying means, thereby generating a high-puritydiluent gas; c) controlling the flow rate of at least onehigh-concentration standard gas; d) mixing the high-purity diluent gasand the at least one high-concentration standard gas, thereby generatinga medium-concentration gas mixture; e) dividing the medium-concentrationgas mixture into a first flow and a second flow; f) selectively mixingthe first flow with a sample gas or a high-purity diluent gas, or amixture thereof, thereby obtaining a standard low-concentration gasmixture; g) controlling the pressure of the sample gas; h) controllingthe pressure of the second flow of the medium-concentration gas mixture;i) controlling the pressure of the standard low-concentration gasmixture.
 2. The process for producing standard gas mixtures according toclaim 1, further comprising:i) dividing said high-purity diluent gasinto a first and second portion; ii) mixing said first portion of thehigh-purity diluent gas and said at least one high-concentrationstandard gas; and iii) diluting said first flow with said second portionof said high-purity diluent gas.
 3. The process for producing standardgas mixtures according to claim 1, further comprising selecting to mixthe first flow with the sample gas, thereby diluting the first flowtherewith, said standard low-concentration gas mixture thereby being alow diluent concentration mixture of high-purity diluent gas, said atleast one high-concentration standard gas, and said sample gas.
 4. Theprocess according to claim 3, further comprising:i) dividing saidhigh-purity diluent gas into a first and second portion; ii) mixing saidfirst portion of said high-purity diluent gas and said at least onehigh-concentration standard gas; and iii) diluting saidlow-concentration gas mixture with said second portion of thehigh-purity diluent gas.
 5. The process for producing standardlow-concentration gas mixtures according to claim 1, furthercomprising:i) dividing said high-purity diluent gas into a first andsecond portion; ii) mixing said first portion of the high purity diluentgas and said at least one high-concentration standard gas; and iii)diluting the first flow with said diluted sample gas.
 6. The processaccording to claim 1, further comprising temporarily stopping the flowof the high-concentration standard gas and of the sample gas in order togenerate the high-purity diluent gas, as the standard gas mixture. 7.The process according to claim 1, further comprising temporarilystopping the flow of the high-concentration standard gas in order togenerate a diluted sample gas as the standard gas mixture.
 8. Theprocess according to claim 1, further comprising restricting the flow ofthe high-purity diluent gas.
 9. The process according to claim 1,further comprising restricting the first flow of themedium-concentration gas mixture.
 10. The process according to claim 1,further comprising restricting the flow of the sample gas.
 11. Theprocess according to claim 1, further comprising restricting the flow ofthe first portion of the high-purity diluent gas.
 12. The processaccording to claim 11, further comprising restricting the flow of thesecond portion of the high-purity diluent gas.
 13. The process accordingto claim 1, comprising a plurality of successive steps wherein the flowpart of the medium-concentration gas mixture is mixed with a portion ofthe high-purity diluent gas to generate a further medium-concentrationgas mixture, the first flow of the last further generatedmedium-concentration gas mixture being finally selectively mixed withthe sample gas or the diluent gas or a mixture thereof to generate thestandard low-concentration gas mixture.